The application of extrudable materials to substrates to form protective films, penetrating films, films with beneficial optical, electrical, aesthetic or physical properties, or for adhesive bonding is widely used. Such practices include the use of varnishes to protect wood surfaces, paints to protect and embellish wood, plastic, ceramic or metallic surfaces, and the use of hot-melt and solvent-based adhesives to bond a wide variety of substrates. Examples of extrudable adhesive use include the bonding of metallic, fabric, foam, wood, leather, and plastic substrates in the assembly of such products as furniture, packaging, automotive sub-assemblies, wooden and metallic windows, trade show exhibits and point-of-purchase displays, electrical components, apparel, luggage, and more.
Many treatments are known to affect the joining of an applied extrudable material to a substrate. Many of these include the use either of chemical reagents to pre-treat the substrate, or the use of laser irradiation either as a pre-treatment (U.S. Pat. No. 4,931,125 to Volkmann et al. and U.S. Pat. No. 4,644,127 to LaRocca), a post-treatment (U.S. Pat. No. 4,861,404 to Neff and U.S. Pat. No. 4,636,609 to Nakamata), or simultaneous with the application of the material to the substrate (U.S. Pat. No. 5,348,604 to Neff).
U.S. Pat. No. 4,931,125 to Volkmann et al. describes a method for pre-treatment of components using a laser beam to create projections and/or depressions in the substrate. This treatment is limited in the types of substrates to which it can be applied, and is generally useful only for non-porous substrates. Also, because of the multiple processes (pre-treatment, followed by bonding) required by this method, it may be expensive to implement in certain industrial environments.
U.S. Pat. No. 4,636,609 to Nakamata teaches the joining of two different kinds of solid synthetic resins, wherein the laser irradiation is used to melt together the two dissimilar resins. This method involves the direct fusion of dissimilar solid synthetic substrates only, and requires specific physical and optical properties for the combination of substrates that significantly limit the range of substrates that may be used.
U.S. Pat. No. 4,644,127 to La Rocca uses a laser to assist in the bonding of metallic pieces. This method teaches the melting of the applied metal by the laser beam prior to its application to the substrate surface, and therefore the substrates are limited to metallic substrates and the applied materials are limited to gas streams containing powdered metals.
The method of U.S. Pat. No. 4,861,404 to Neff involves the transfer of heat from a laser directly to the bulk extrudable material for purposes of heating the material. However, this requires extremely high energy densities, since the energy is not concentrated at the interface between the substrate and the material, where the deposited energy has its greatest effect, but is distributed throughout the material. Furthermore, because the extrudable material is heated in bulk, this greatly increases the time required for the material to regain structural integrity (the "closing" time), an important factor in many manufacturing applications. In addition, this method requires certain optical properties of the extrudable material that limit the range of its application.
The method of U.S. Pat. No. 5,348,604 to Neff requires that light-energy pass through the extrudable material within the nozzle apparatus. Because of the high energy densities required in the technique, this is generally practical only with light energy from a laser that, as discussed below, is difficult and expensive in many manufacturing environments. Furthermore, this method precludes the use of the light energy which passes through the adhesive from initiating a catalysis of the extrudable materials, such as those used to strengthen certain hot-melt adhesives, since the curing of any adhesive that resides within the nozzle would render the nozzle inoperable. In addition, this method requires special optical properties of the extrudable material that limit its range of applications. Also, this method places limits on the extrusion apparatus for the material that increases the cost and complexity of the apparatus.
The prior art described above generally involves laser irradiation of the extrudable material or the substrate. While lasers excel at providing highly concentrated radiation, high-power lasers tend to be complicated and costly to operate, including YAG lasers, which are often used because of the superior quality of the wavelength of light produced. Furthermore, due to the requirement of precisely orienting and placing the laser mirrors, as well as the use of sophisticated water-cooling mechanisms for certain laser classes, including YAG lasers, which require water-purifiers, heat-exchangers, and refrigerator systems, lasers in industrial environments may require frequent maintenance. Also, many high-power lasers, including YAG lasers, output only a small fraction of the electrical-energy input, requiring large power supplies, waste heat elimination systems, and large power usage for relatively small power applications. In general, high-power lasers are expensive to purchase, operate and maintain. All of these disadvantages make high-power lasers, and the methods that employ them, unsuitable for many industrial applications.
It was our intention to create a method that could use simple and inexpensive devices to enhance the bonding of extrudable materials to a substrate. It was our intention to solve the problems of the prior art that gave rise to the current invention.